The present invention relates to pianos, harpsichords and like instruments in which a plurality of tensioned springs are struck or plucked to produce musical sounds. More particularly, the present invention relates to a back frame which holds the strings in tension in the musical instrument.
In both grand pianos and upright pianos there is a large frame assembly which supports the strings adjacent the hammers and holds the strings in tension over a bridge or bridges on the sounding board. The assembly traditionally includes a large metal plate defining a framework over which the strings are stretched from one side to the other, a wrest plank with a plurality of rotatable tuning pins mounted in the plank and projecting through the plate and a wooden frame connected with the wrest plank and the plate and reinforcing the plate against the large tensioning load of the strings. The sounding board is generally positioned between the plate and the wooden frame with the strings stretched over the board-mounted bridges.
The wooden frame is referred to hereinafter as the back frame. The term "back" in the case of an upright piano refers to the rear of the piano opposite from the keyboard at the front, and in the case of a grand piano or harpsichord, the back refers to the area behind or below the sounding board at the bottom side of the case.
The back frame traditionally includes a plurality of beams which extend from one side of the frame to the other in a direction generally parallel to the tensioning load of the strings and which are subjected to a bending moment due to the tensioning load. The beams are massive hardwood posts that in upright pianos or consoles may be three or four feet in length and have cross-sections of 3 inches by 2 inches.
The hardwood posts of such size are only a few of the many pieces which must be assembled and glued together, sometimes with additional bolts, to form the back frame, and because of the size of the frames and the fact that they must be made from a durable hardwood such as oak, they are relatively expensive and increase the cost of the instrument. The wood is also sensitive to moisture which may cause distortion and change the pitch of the tensioned strings, and the glued joints between the assembled pieces of the frame may weakened and fail with age and repeated exposure to moisture.
It is accordingly a general object of the present invention to provide an improved back frame for a piano or like instrument which can be manufactured from materials that are readily available and at lower cost. It is an object to the invention to provide a back frame having a laminated, composite structure with high stiffness or resistance to creep and distortion and the reliability inherent in an integrated design.